1. Principles of Heredity
Lab Activity 36
Principles of Heredity
Portland Community College
BI 233

2. Terminology of Chromosomes
Homologous chromosomes: A pair, of which you get one from mom, and one from dad.
Example: the pair of chromosomes 21 are homologous to each other
Sex Chromosomes: The X and Y
These are homologous to each other
Autosomal Chromosomes: The other 22 pairs of chromosomes that do not determine gender
Karyotype: A chart of the chromosomes arranged in homologous pairs.

3. Karyotype of Human Chromosomes
22 pairs of autosomes
1 pair of sex chromosomes

5. Terminology of Genes: Alleles
Allele: Alternative form of a gene at the same locus on homologous chromosomes
Homozygous: Two alleles controlling a single trait are the same
Heterozygous: The two alleles for a trait are different
Dominant: An allele masks or suppresses the expression of its partner
Recessive: The allele that is masked or suppressed

7. Terminology Genotype: the genetic makeup
Phenotype: the way one’s genotype is expressed
Punnett square
Method of showing 4 possible genetic combinations in offspring of 2 individuals

8. A a AA Aa A a Aa aa Mother: Aa Father Aa Homozygous Dominant
Heterozygous A a Aa
Heterozygous aa
Homozygous Recessive

9. Dominant and Recessive Genes
Homozygous: The person has 2 copies of the dominant or 2 copies of the recessive gene (gets one from each parent)
Heterozygous: The person has 1 copy of the dominant and 1 copy of the recessive gene
Genotype: Aa
Phenotype: Dominant Characteristic A

10. Example: Eye Color Dominant and Recessive
Dominant: Brown (B)
Recessive: Blue (b)
Parents:
Mom blue eyed (bb),
Dad: homozygous brown eyed (BB)
100% of children will be heterozygous (Bb) with brown eyes
B B b Bb Bb Bb Bb

11. Example: Eye Color Dominant and Recessive
Dominant: Brown (B)
Recessive: Blue (b)
Parents:
Mom blue eyed (bb),
Dad: heterozygous brown eyed (Bb)
50% of children will be heterozygous (Bb) with brown eyes
50% will be homozygous recessive (bb) with blue eyes
B b b Bb bb Bb bb

12. Dominant and Recessive

13. Why Marrying Your Cousin is Bad!!!
Inbreeding causes recessive alleles to become homozygous more often.
If the recessive allele contains a genetic disease, it will show up in these children at a higher rate than in the normal population.
Examples:
Tay-Sachs disease occurs primarily among Jews of Eastern European descent

15. Incomplete Dominance
Heterozygous individuals have a phenotype intermediate between homozygous dominant and homozygous recessive
Sickling is a human example when aberrant hemoglobin (Hb) is made from the recessive allele (s)
SS = normal Hb is made
Ss = sickle-cell trait (both aberrant and normal Hb is made)
ss = sickle-cell anemia (only aberrant Hb is made)

16. Incomplete Dominance

17. Incomplete Dominance

18. Incomplete Dominance

19. Multiple-Allele Inheritance
Genes that exhibit more than two alternate alleles
ABO blood grouping is an example
Three alleles (IA, IB, i) determine the ABO blood type in humans
IA and IB are codominant (both are expressed if present), and i is recessive
Rh factor is complete dominance

20. Sex-Linked Inheritance
Inherited traits determined by genes on the sex chromosomes
X chromosomes bear over 2500 genes; Y carries about 15
X-linked genes are:
Found only on the X chromosome
Typically passed from mothers to sons
Never masked or damped in males since there is no Y counterpart

21. Example: Color blindness Sex-Linked Inheritance
Dad is color blind (X°Y)
Mom is homozygous normal (XX)
No sons affected
All daughters are carriers
X° Y X
XX° XY
XX° XY

22. Example: Color blindness Sex-Linked Inheritance
Dad is normal (XY)
Mom is a carrier (X°X)
50% of sons affected
50% of daughters are carriers
X Y X° X
X°X
X°Y XX XY

23. Example: Color blindness Sex-Linked Inheritance
Dad is normal (XY)
Mom is color blind (X°X°)
100% of sons affected
100% of daughters are carriers
X Y X°
X°X
X°Y
X°X
X°Y

24. X-Chromosome Inactivation
Females have double dose of X chromosome in all cells
One X chromosome is randomly & permanently inactivated early in development
Visible as dark-staining Barr body easily seen in nucleus of neutrophils as “drumstick”
Tightly coiled even in interphase cell

25. Example: X-Chromosome Inactivation

26. Polygenic Inheritance
Depends on several different gene pairs at different loci acting in tandem
Results in continuous phenotypic variation between two extremes
Examples: skin color, eye color, and height
Although we think of eye color as simple dominant/recessive, there are many genes that code for eye color, which is why your eyes are not usually the exactly the same color as either of your parents.

27. Genomic Imprinting
Genomic imprinting is the differential expression of a gene depending on whether it is inherited from the mother or the father
Examples:
Igf2: Insulin-like growth factor 2
Only the paternal gene is expressed
Affects birth weight of human infants
Prader-Willi and Angelman syndromes

28. Genomic Imprinting Prader-Willi and Angelman Syndromes
Caused by a small deletion on Chromosome 15
Prader-Willi Syndrome: Paternal inheritance
Small hands and feet, Short stature
Voracious appetite
Mental retardation
Angelman Syndrome: Maternal inheritance
Uncontrolled muscle movement
Feeding issues (poor eaters)
Very happy demeanor (laugh all the time)
Unusual seizures

29. Cytoplasmic Inheritance
A zygote inherits nuclear genes from both parents
Cytoplasmic organelles are inherited only from the mother
Mitochondria have their own DNA
Contains 37 genes (nuclear DNA has 30,000 genes)
Always passed from mother to all children

30. Abnormal Chromosomes: Trisomy
Trisomy: Having 3 copies of a chromosome (or part of a chromosome) instead of 2
Can be caused by nondisjunction during Meiosis II when the sister chromatids are supposed to separate.
Can happen during prophase I crossing over (partial duplication of a chromosome)
Can happen during mitosis
Usually lethal

31. Trisomy 21 Down’s syndrome Severe: 3 copies of chromosome 21
Less severe: one chromosome 21 has an “extra” piece, so only some of the genes are in triplicate
Less severe: if the defect happened during mitosis in the early embryo resulting in only some cells carrying 3 copies

32. Trisomy 18: Edward’s Syndrome
Severely retarded
Usually fatal shortly after birth.

33. Trisomy XXX 1 in 1,000 newborn girls
Many girls and women with Triple X have no signs or symptoms. Signs and symptoms vary a lot between individuals, but can include:
Increased space between the eyes
Tall stature (height)
Small head
Learning disabilities
Delayed puberty
Infertility
Mental retardation

34. XXY Klinefelter's Syndrome
1 in 1,000 male births
Sterility
Breast development
Incomplete masculine body build
Social and/or school learning problems

35. Monosomy: XO Turner Syndrome
Females with only one X
Short stature
Lack of ovarian development
Webbed neck
Arms turn out slightly at the elbow
Mentally normal

36. Pleiotropy
The control by a single gene of several distinct and seemingly unrelated phenotypic effects.
Example: PKU (phenylketonuria).
This disease causes mental retardation and reduced hair and skin pigmentation.
The cause is a mutation in a single gene that codes for the enzyme phenylalanine that converts the phenylalanine to tyrosine
The mutation results in no or reduced conversion of phenylalanine to tyrosine, and phenylalanine concentrations increase to toxic levels, causing damage at several locations in the body.

37. Pedigree
A pedigree is a diagram of family relationships that uses symbols to represent people and lines to represent genetic relationships.
These diagrams make it easier to visualize relationships within families, particularly large extended families.
Pedigrees are often used to determine the mode of inheritance (dominant, recessive, etc.) of genetic diseases.

38. Factors to Consider in Pedigrees
Is the trait located on a sex chromosome or an autosome?
Autosomal – not on a sex chromosome
Sex Linkage – located on one of the sex chromosomes
Y-linked - only males carry the trait.
X-linked (recessive) - sons inherit the disease from normal parents
How is the trait expressed?
Dominant - the trait is expressed in every generation.
Recessive - expression of the trait may skip generations.

39. Pedigree Diagrams

40. Autosomal Dominant Inheritance
A disorder appears in several generations of a family.
Affected parents have a 50% risk of an affected child with each pregnancy.
Points to consider:
1. Affected individuals have at least one affected parent
2. The phenotype generally appears every generation
3. Two unaffected parents only have unaffected offspring

41. Autosomal Recessive Inheritance
Disorders often appear in only one generation of a family.
Carrier couples have a 25% risk of an affected child with each pregnancy.
Points to consider:
1. Unaffected parents can have affected offspring
2. Affected progeny are both male and female

42. Example: Albinism
Expressed in both sexes at approximately equal frequency, therefore it is autosomal.
Not expressed in every generation, therefore it is recessive.

43. Albinism: Genotype of the Affected Individuals
Assign codes for the alleles.
Code “A” for the dominant normal allele.
Code “a” for the recessive allele for albinism.
Affected individuals must be homozygous for “a.”
First generation parents must be “Aa” because they have normal phenotypes, but affected offspring. Aa Aa aa aa aa aa aa

44. Albinism: Genotype of the Normal Individuals
Normal individuals must have at least one “A.” Aa Aa A aa A aa A aa A aa aa A A A A A A A A

45. Albinism: Parent-Offspring Relationships
“1” must transmit “a” to each offspring.
The “A” in the offspring must come from the father.
Normal father “2” could be either heterozygous or homozygous for an “A?” Aa Aa 1 2 A aa A aa A aa A? aa aa A A Aa Aa Aa Aa Aa Aa

46. X-linked Inheritance X-linked diseases are almost always recessive
Male to male transmission of X-linked disorders is not seen.
Carriers are designated with a dot.

47. Example: Hemophilia
In this pedigree, only males are affected, and sons do not share the phenotypes of their fathers.
Thus, hemophilia is linked to a sex chromosome–the X.
Expression of hemophilia skips generations.
Thus, it is recessive.

48. Hemophilia: Expression of the Female Sex Chromosomes
All females are XX.

49. Hemophilia: Genotype the Affected Individuals
Assign codes for the alleles.
Code “H” for the recessive hemophilia allele.
Affected individuals must have an “H” on an X chromosome.

50. Y-Linked Inheritance Only males are affected.
All sons of an affected father have the trait.
Thus, the trait is Y-linked.

51. Cytoplasmic Inheritance
Always passed from mother to all children
Never passed through males to their offspring.

52. The End
The End